Methodology for the In Vivo Measurement of the Δ⁹‐Desaturation of Myristic, Palmitic, and Stearic Acids in Lactating Dairy Cattle

Abstract

There is limited methodology available to quantitatively assess the activity of the Delta9-desaturase enzyme in vivo without chemically inhibiting the enzyme or using radioactively labeled substrates. The objective of these experiments was to develop methodology to determine the incorporation and desaturation of 13C-labeled fatty acids into milk lipids. In a preliminary experiment, 3.7 g [1-13C]myristic acid ([1-13C]14:0), 19.5 g [1-13C]palmitic acid ([1-13C]16:0), 20.0 g [1-13C]stearic acid ([1-13C]18:0) were combined and infused into the duodenum of a cow over 24 h. In a following experiment, 5.0 g [1-13C]14:0, 40.0 g [1-13C]16:0, and 50.0 g [1-13C]18:0 were infused into the abomasums of separate cows as a bolus over 20 min or continuously over 24 h. Milk fat was extracted using chloroform:methanol. Fatty acids were methylated, and fatty acid methyl esters (FAME) were converted to dimethyl disulfide derivatives (DMDS). The FAME and DMDS were analyzed by gas chromatography mass spectrometry. In the preliminary experiment, 13C enrichment in 14:0 but not 16:0 or 18:0 was observed. When dosage amounts were increased in the following experiment, peak enrichments from the bolus infusion were observed at 8 h. Enrichments for continuous infusion peaked at 16 h for 14:0 and 18:0, and at 24 h for 16:0. The Delta9-desaturase products of these fatty acids were estimated to be 90% of cis-9 14:1, 50% of cis-9 16:1, and 59% of cis-9 18:1. This study demonstrates that 13C-labeled fatty acids may be utilized in vivo to measure the activity of the Delta9-desaturase enzyme.

Role of trans fatty acids in the nutritional regulation of mammary lipogenesis in ruminants

“…Between 49% and 60% of 18:0 available for mammary TAG synthesis is desaturated with the extent of endogenous conversion accounting for ca. 60% of the OA secreted in milk (Enjalbert et al, 1998;Mosley and McGuire, 2007;Glasser et al, 2008b). Activity of SCD also contributes to approximately 90% of cis-9 14:1 and 50% to 56% of cis-9 16:1 in bovine milk fat (Mosley and McGuire, 2007).…”

“…60% of the OA secreted in milk (Enjalbert et al, 1998;Mosley and McGuire, 2007;Glasser et al, 2008b). Activity of SCD also contributes to approximately 90% of cis-9 14:1 and 50% to 56% of cis-9 16:1 in bovine milk fat (Mosley and McGuire, 2007). A number of other saturated acyl CoA also serve as substrates for SCD including 10:0, 12:0, 14:0, 15:0 and 17:0 (Bauman and Davis, 1974;Shingfield et al, 2008c).…”

Effect of replacing calcium salts of palm oil distillate with incremental amounts of conventional or high oleic acid milled rapeseed on milk fatty acid composition in cows fed maize silage-based diets

“…A lower concentration of cis-9 16:1 can be attributed to decreased supply of 16:0 available for conversion to cis-9 16:1 via the action of D 9 -desaturase in the mammary gland. Use of 13 C-labelled FA substrates indicate that around 50% cis-9 16:1 secreted in milk is synthesised endogenously via the action of D 9 -desaturase on 16:0 (Mosley and McGuire, 2007).…”

Genetic variability of milk fatty acids

“…According to Feng et al (2007) and Lock and Garnsworthy (2003), the C14 desaturase index is considered as the best indicator of desaturase activity. In fact, 90% of C14:1 cis-9 is the result of SCD activity (Mosley and McGuire 2007). The total concentrations of MUFAs and CLA should increase in fat if SCD activity rises, improving in this way the nutritional quality of milk.…”